This invention relates to an optical type information reproducing device for laser discs such as a compact disc of the Compact Disc Digital Audio System and a video disc and, more particularly, to a device of such type capable of controlling a laser beam output of a laser pickup so as to maintain the level of a reflected or transmitted laser beam at a constant value.
In an optical type disc playback device, an original signal is reproduced by shaping the waveform of a high-frequency signal reproduced from a disc at a predetermined threshold level. For eliminating an error in reading information recorded on a disc, therefore, it is important to maintain the level of the high-frequency reproduced signal at a constant value.
The construction of a prior art optical type information reproducing device (optical type digital audio disc playback device such as a Compact Disc playback device) is shown in FIG. 1. In FIG. 1, a laser pickup 10 includes a semiconductor laser 12, PIN diodes 14 for reproducing a signal and a laser output detecting PIN diode 15 for monitoring an output level of the laser 12. Laser beam 16 projected from the semiconductor laser 12 is reflected by a disc 18 and received by the diodes 14. An output signal of the diodes 14 is applied through a high-frequency amplifier 20 to a level adjusting resistor 22 in which an output signal of the amplifier 20 is adjusted in its level. An output signal of the level adjusting resistor 22 is applied to an audio signal reproducing circuit 24 for reproduction processings such as digital-to-analog conversion and thereafter is outputted as an audio signal.
The diode 15 receives laser beam 16' outputted from the semiconductor laser 12 and applies a laser signal responsive to the laser beam 16' to a laser power control circuit 26. The laser power control circuit 26 is provided with a power voltage -V and functions to maintain the laser beam output from the laser 12 at a constant level by controlling a voltage (i.e., .varies. operation current) applied to the semiconductor laser 12 in response to the laser signal derived from the diode 15. This constant level is set by an output setting variable resistor 28.
The optical type information reproducing device as shown in FIG. 1 is disadvantageous in that it cannot cope with fluctuation in the level of the high-frequency reproduced signal due to irregularity of the disc surface since the control of the laser beam output in this type of device is made on the basis of detection of the laser beam 16' by the monitoring diode 15 and not on the basis of detection of reflected laser beam from the disc 18. Besides, this prior art device requires the diode 15 provided exclusively for the control of the laser beam output with a resulting complicated structure of the device and high manufacturing cost. It is also a disadvantage of the device that, since the high-frequency signal reproducing system and the laser output control system are separated from each other, adjusting of the level of the reproduced signal against irregularities of the laser pickup 10 and relating circuits is required and for this purpose, the level adjusting resistor 22 must be provided.